1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques, etching applications and thin-film metrology.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to determine features of the substrate, such as its layer thickness, critical dimension (CD) or overlay, a beam is reflected off the surface of the substrate, for example at an alignment target, and an image is created on a camera of the reflected beam. By measuring the properties of the reflected beam, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties.
One way of measuring the properties of a substrate is by diffracting a polarized beam off a grating that is present on the surface of the substrate and imaging the diffracted spectrum of the polarized beam on a detector array. The polarization state of the measurement beam must be controlled so that the polarization state is not altered by imperfections of the sensor optics. One way of doing this is shown in FIG. 2. Two sources, P and S, are merged using a polarizing beamsplitter PBS. The use of the PBS ensures that the light from the P and S sources are S and P polarized respectively (i.e. the polarization of the beams is perpendicular and parallel to the plane of incidence of the PBS respectively). In order to be able to measure the diffracted spectrum of each polarized beam on the same camera CCD, a shutter (not shown) is provided at each of the sources. The two beams p,s are alternately shuttered so that at any one moment in time, only one of the two polarizations is passing through the system. This requires a lot of quickly moving parts and the alternating between the two polarizations increases the time taken for tests carried out on the substrate, causing throughput time to be increased.